Conventional rosin and hydrocarbon-based resins are prepared as relatively low viscosity resins in stirred reaction vessels. Such resins are usually highly branched and possess very broad molecular weight distributions. The product may even contain some fraction of crosslinked gelled structures, but this fraction is typically limited in quantity by virtue of the inability of a stirred tank vessel to handle the very high molten viscosity and non-Newtonian behavior that normally accompanies the presence of a gelled resin structure.
Lithographic ink vehicles or varnishes may be prepared from existing resins by dissolving/dispersing the resins in lithographic ink solvents and then subjecting the mixture to a so-called “gelling” reaction using an aluminum-based gelling agent. The role of the aluminum compound in the preparation of lithographic ink vehicles has typically been that of controlling the rheological properties of the ink vehicle. Known aluminum gelling compounds for use in preparing ink vehicles may be classified as: 1) aluminum soaps, 2) aluminum alkoxides, 3) chelated alkoxides and 4) oxyaluminum acylates. However, aluminum compounds require the generation or existence of an aluminum hydroxyl functionality to form the ultimate rheology or gel structure. The resulting gel structure is the result of relatively weak coordinate covalent or hydrogen bonding of the aluminum hydroxyl species with the binder resin system.
Since the coordinate covalent and hydrogen bonds formed with the aluminum gelling agents are relatively weak bonds, typically only about 5 to 10% as strong as covalent bonds, the gel structure afforded by such bonds is substantially degraded under the high shear conditions associated with modern lithographic printing. While some degree of thixotropic behavior is important for successful printing, there are limits to the degree of viscosity changes which can be tolerated before unwanted side effects emerge. Modern high speed printing presses rapidly degrade conventional ink vehicles to a significant degree. One consequence of the loss of viscosity in an ink vehicle is the development of misting, which is more pronounced in high speed presses leading to potential losses in print quality and degradation of the work environment. Misting may occur as a result of ink shear caused by high press speeds thereby forming minute droplets or mists of ink. Inks which maintain higher viscosities under high shear printing conditions are less likely to mist.
Yet another undesirable aspect of the use of gelling agents is that such agents are typically the most expensive ingredient in the varnish formulation on a weight percent basis. Hence, the use of organo-aluminum gelling agents presents a definite economic liability.
Additionally, the lithographic printing industry is shifting to the use of higher molecular weight/higher solution viscosity “self structuring” ink resins to improve the press performance of inks during high speed press operations. New generation lithographic printing presses are capable of achieving printing speeds in excess of 3000 feet/minute. In order for these ink vehicles to be effective in high speed printing presses, increased shear resistance of the vehicles with the printing equipment is important.
Japanese patent application JP5-171089 is directed to ink varnishes which exhibit high gloss and improved setting and drying properties as a result of crosslinking a portion of the polar groups of the binder resin before the resin is gelled with a metal gelling agent. The agents used to reduce the number of polar groups in the resin are certain amino compounds; preferably melamine resin or benzoguanamine resin containing at least one functional group selected from methylol, alkoxy and imino groups, and the reaction is conducted below the softening point of the binder resin and before all of the resin is dissolved. The gellation reaction is conducted separately from the crosslinking reaction once the polar groups have been sufficiently cross-linked.
U.S. Pat. No. 5,763,565 discloses a process for making gelled ink resins with improved properties. In one embodiment a rosin-based or hydrocarbon-based resin is mixed with an organic solvent and reacted with a crosslink agent under conditions sufficient to produce a substantially covalently cross-linked gelled resin.
A consequence of the loss of viscosity of an ink resin resulting from excessive shear is a loss of print sharpness. When a gelled ink resin is sheared to the extent that flow becomes significant, the printed dot is diffused thereby yielding excessive “dot gain” or poor print quality.
Because of the trend toward higher speed printing presses and the continuing need to improve the ink application process and print quality, there continues to be a need for improved resins for use as ink vehicles in the printing industry and improved processes for making the resins. An object of the invention is to provide shear resistant ink vehicle compositions which reduce misting and dot gain on high speed printing machines.
Still another object of the invention is to provide an in situ process for making shear resistant vehicle compositions of the character described with readily available, relatively inexpensive materials.
Another object of the invention is to provide a process for making viscous shear resistant vehicle compositions which enables improved control over the properties of the resins and which enables improved uniformity in the properties of large quantities of resin.
Yet another object of the invention is to provide a process for making resin compositions for high speed printing applications which maintain their viscosity even under the vigorous conditions associated with high speed printing.
An additional object of the invention is to provide a process for making shear resistant vehicles of the character described which is cost effective and uncomplicated and does not require the use of highly specialized or complicated equipment.